2:15 PM - 2:30 PM
[SSS03-03] Offshore seismic activity in the vicinity of Monday Seamount in October 2006 from detailed epicenter distribution
Keywords:Ocean seismic activities, Submarine volcanoes, Waveform data analysis
A Mw=6.4 earthquake, possibly related to submarine volcanism, occurred in October 2006 in the vicinity of Monday Seamount, located in the Izu-Bonin-Mariana arc. At the time, a BBOBS (Broadband Ocean Bottom Seismometer) array was deployed in the vicinity of Monday Seamount. Although the seismic sites are located farther (~100-1000 km) than the onshore volcano monitoring situation, the mainshock occurred inside the array. We, therefore, expect that the detailed aftershock distribution is delineated from the dataset of the array. Following Aoyama et al. (2023, JpGU), who developed a new method, this study locates the aftershocks careful reading of the arrival time and data selection. Next, we relocate the detailed distribution of the epicenter of aftershocks. Moreover, we discuss what volcanic activity occurred to illustrate the distribution of this result.
The Japan Meteorological Agency's (JMA) catalog contains ~30 aftershocks within about a week. The aftershocks located by JMA are extremely widespread in the east-west direction, with a spatial extent too wide for aftershocks of a Mw=6.4 earthquake. These earthquake locations motivated us to relocate the aftershocks precisely to understand the mechanics of the Mw=6.4 mainshock better. However, there are two obstacles to achieving our goal. One is that the detailed crustal structure of this region is not well understood like many oceanic regions. The other is that the waveforms of submarine seismometers are complex, making it challenging to read arrivals and identify the phase correctly. Aoyama et al. (2023, JpGU) detected the P-wave arrivals of the mainshock and aftershocks with high accuracy using a method developed by Baillard et al. (2014). Moreover, we measured the differential travel times between two seismic sites unaffected by seismic velocity structure between the hypocenter and seismic sites. We then excluded anomalous measurements by visual inspection. Through these procedures, we relocated epicenters of ~45 aftershocks, including not only those recorded in the JMA catalog but also those with amplitudes above a certain threshold.
The relocated epicenters are distributed around the edifice of Monday Seamount. We also found a change in epicenter locations approximately 8 hours after the mainshock. The epicenters were randomly distributed mainly in the edifice of Monday Seamount during the first ~8 hours after the mainshock. However, after ~8 hours after the mainshock, aftershocks are concentrated in a rift zone west of Monday Seamount. This epicenter distribution is likely due to dike intrusion at the rift zone west of the edifice. The dike intrusion is likely triggered by the normal-faulting earthquake, whose stress perturbation made the stress field more extensional to favor a dike intrusion. By this stress perturbation, we can interpret that the dike intrusion led to the concentration of aftershocks in the rift zone.
The Japan Meteorological Agency's (JMA) catalog contains ~30 aftershocks within about a week. The aftershocks located by JMA are extremely widespread in the east-west direction, with a spatial extent too wide for aftershocks of a Mw=6.4 earthquake. These earthquake locations motivated us to relocate the aftershocks precisely to understand the mechanics of the Mw=6.4 mainshock better. However, there are two obstacles to achieving our goal. One is that the detailed crustal structure of this region is not well understood like many oceanic regions. The other is that the waveforms of submarine seismometers are complex, making it challenging to read arrivals and identify the phase correctly. Aoyama et al. (2023, JpGU) detected the P-wave arrivals of the mainshock and aftershocks with high accuracy using a method developed by Baillard et al. (2014). Moreover, we measured the differential travel times between two seismic sites unaffected by seismic velocity structure between the hypocenter and seismic sites. We then excluded anomalous measurements by visual inspection. Through these procedures, we relocated epicenters of ~45 aftershocks, including not only those recorded in the JMA catalog but also those with amplitudes above a certain threshold.
The relocated epicenters are distributed around the edifice of Monday Seamount. We also found a change in epicenter locations approximately 8 hours after the mainshock. The epicenters were randomly distributed mainly in the edifice of Monday Seamount during the first ~8 hours after the mainshock. However, after ~8 hours after the mainshock, aftershocks are concentrated in a rift zone west of Monday Seamount. This epicenter distribution is likely due to dike intrusion at the rift zone west of the edifice. The dike intrusion is likely triggered by the normal-faulting earthquake, whose stress perturbation made the stress field more extensional to favor a dike intrusion. By this stress perturbation, we can interpret that the dike intrusion led to the concentration of aftershocks in the rift zone.